56 SYSTEMATIC ARRANGEMENT 



GROUP THE FIRST. 



The lateral lobes of the rings of the body are situated in the same plane, and do not curve or 

 bend downwards, but terminate towards the posterior part in a more or less prominent point, some- 

 times very long, tchich forms a somewhat obtuse angle in its principal direction with the direction 

 of the lobe. 



Of this group there are further subdivisions, which may readily be distinguished from 

 one another, being founded on the presence or absence of a large caudal shield. 



Division A. 



Trilobites with a simple, large, caudal shield {not much smaller than the cephalic shield), 

 the axis of which is many-jointed, and equals, or even exceeds, the length of the body. Ogygiid^. 

 To this group belong two genera, Trinucleus (with six rings) and Ogygia (with eight rings). 



Genus 1. — Trinucleus, Murchison (Cryptolithus, Green). 



Cephalic shield almost semicircular, with a margin which is dilated all round and 

 punctated, and with posterior angles terminating in long spines ; the central glabella very 

 convex, much contracted at its posterior part before the margin ; without lateral lobes or 

 furrows. 



The eyes and the facial suture I have not yet myself been able to examine.* The body 

 is short, the six rings narrow, scarcely half as broad as the lateral lobes, the latter with a 

 distinct diagonal furrow, and with a fine short angle. 



Caudal shield triangular ; the axis articulated in six or more jointed, the sides without 

 radial furrows ; the margin reflexed and acutely angular. 



Loccdity. — In the lower and middle Silurian strata. 



Remark. — Several species are known, from the lower Silurian strata of England, North America, 

 Sweden, and Bohemia. Some authors enumerate only five body-riugs, probably by mistake, or in 

 consequence of the defective preservation of the individual. 



1. T. Caractaci : Limbo scuti cephalici orbicular!, conceutrice punctato ; angulis posticis subrectis, 

 longe spinosis; caudse basi annulata, apice scuti acuto. Long. %-\]ii". Plate I, Fig. 1. 



• Ref—T. Caractaci, MuRCHisoN, Sil. Syst. pt. ii, p. 659, Plate XXIII, Fig. I, a, b, c, d, e. 



Brongn. Crust, foss. Plate IV, Figs. 6, 7, A, B, C, p. 145. Lhwyd, Phil. Trans. 



vol. XX, p. 243, Plate, Fig. 8. Bigsby, Jn/ials of the Lye. of Nat. Hist, of New York, 



i, 214, Plate XV, Fig. 1. Emmr. Diss. 51. 6. Milne Edw. Crust, iii, 331. 1. 



Portlock, Report, 262, Plate I, B, Fig. 3-7. 



Cephalic shield nearly semicircular, broader than long, the enlarged margin covered 



with five to six concentrical rows of little pits, from which rise small wart-like prominences ; 



posterior angles oblique and oj)pressed, each terminating in a long pointed spire as long as 



* Dr. Loven (Ofvers of Kongl. Vetensk. Ak. Förh. 1845, No. 4) describes the facial suture as 

 running close to the margin of the cephahc shield, turning inwards at the posterior angles, and inter- 

 secting'' the posterior margin of the shield at about the middle. In accordance with this statement, 

 such an arrangement has been indicated in the figure of Trinucleus Caractaci. 



